An environmental activist who drove a Prius long before Hollywood jumped on the hybrid bandwagon, Cameron Diaz is greener than Fiona, the feisty ogress she plays in "Shrek Forever After", the fourth and final film in the animated series. “The word ‘green’ is a way to identify a certain mentality, obviously, but I think that it’s important for everybody to be, because we only have one planet and no one can live without clean air, clean soil and clean water. None of us can live without that,” says Diaz (pictured above and left). “It affects all of us, every human being. The only thing 6 and a half billion people share on this planet is the planet. It just makes sense to take care of your life force and the resources that sustain you.”

Opening May 21, the fourth Shrek movie, in which Shrek, disillusioned by domesticity and missing his fearsome old self, makes a deal with devious Rumpelstiltskin that thrusts him into an alternate reality in which Fiona is the fierce leader of the ogre resistance whose heart Shrek must win all over again.

“She’s always been a warrior, fought for the love that she has for Shrek and her family and friends. Her responsibilities are just a little bit more obvious this time,” compares Diaz, who believes the film will appeal to all ages. “Anyone who watches it can understand exactly what Shrek and Fiona are going through relevant to their own lives.”

Diaz, who’ll next team with Tom Cruise in "Knight and Day", out June 25, wouldn’t rule out reprising Fiona in the future, but for now the franchise will focus on the character Puss in Boots, Antonio Banderas’ suave kitty-turned-fat cat (literally) in "Forever". Due in 2011, the prequel will pay homage to the style of Sergio Leone spaghetti westerns and cover the fearless feline’s past. “You’ll see why he became an ogre killer,” Banderas noted at the "Shrek Forever After" premiere.

Accompanying him on the green (what else?) carpet was his wife Melanie Griffith, who told us that the Banderas family recycles at home. “We do everything we can without changing the whole house,” said Griffith, who’s looking for a good movie or series role. “I’d kind of like to do a TV show,” she said.

“I think we ended on a high note. It’s all good,” said Eddie Murphy of his last turn as Shrek’s song-happy sidekick Donkey. “I wish that I could say that I was a green guy,” admitted Murphy, who has recycling bins at home but sometimes neglects to use them properly. Fortunately, his children keep him in line. “Every now and then I’ll throw something in there, and the kids are like, ‘Hey! That’s for recycling!’ ”

Walt Dohrn, the "Shrek" story editor who voices villain Rumpelstiltskin, has an equally eco-conscious offspring, his daughter Phoebe, 7. “It’s very important to her. So we make sure we recycle and bring our own bags to the grocery store. I use public transportation whenever I can, especially since I don’t drive,” adds Dohrn. “I take the bus, a car service.”

Also new to the movie are the more recognizable voices of Jon Hamm of "Mad Men", Jane Lynch of "Glee", and Craig Robinson of "The Office" as ogres; Kathy Griffin, Meredith Vieira, and Mary Kay Place as witches; and "American Idol" host Ryan Seacrest (pictured left) as the father of a pushy kid, a two-line role he calls “a giant leap from my cameo in "Knocked Up".

“I am green. I need to get better at it,” confided Seacrest, who recycles at home and is “becoming more and more paperless” in his office. “We’re trying to save some trees by eliminating scripts and papers. Small steps.” Seacrest and the "American Idol" judges voice animated versions of themselves in the season finale of "The Simpsons" on May 23 in advance of the "AI"finale on May 25 and 26 on Fox.

Holly Robinson Peete, (pictured right) who attended the premiere with her family of “big Shrek fans,” told us she prepares trash-free lunches for her kids, and buys only organic food and green products. A finalist for the title of "Celebrity Apprentice", she has a good shot at winning more cash to fight autism via her HollyRod Foundation if she beats Bret Michaels in the May 23 finale. “It was probably one of the hardest things I’ve done,” she said, “It shaved a couple of years off my life, but for the families affected by autism, it was totally worth it.”